Method and apparatus for producing and controlling electron emission



6 1 1. m 5 e i h 7 S 1 9 O, a v e h S Jan. 16, 1962 A. R. VON HIPPEL METHOD AND APPARATUS FOR PR CONTROLLING ELECTRON EMISSION Filed March 15, 1954 FILAMENT CURRENT 9 AMPS 'F" CENTER ELECTRON Fig. I

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ATTORNEYS Jan- 1 1962 A. R. VON HIPPEL 3,017,516

METHOD AND APPARATUS FOR PRODUCING AND CONTROLLING ELECTRON EMISSION Flled March 16, 1954 2 Sheets-Sheet 2 FILAMENT CURRENT= I0.0 AMPS. 900 V PHOTOCURRENT PULSE (AMPERES) I I l l l I F I O 20 4O 6O 80 I00 I20 I40 I60 DARK TIME INTERVAL TO ELECTRONIC AMPLIFIER VARIABLE DC Flg. 9

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0 ARTHUR R. VON HIPPEL I By g M, M ,LULIZTL ATTORNEYS I United States Patent METHOD AND APPARATUS FOR PRODUCING AND CONTROLLING ELECTRON EMISSION Arthur R. von Hippel, Weston, Mass, assignor to Research Corporation, New York, N.Y., a corporation of New York Filed Mar. 15, 1954, Ser. No. 416,351 14 Claims. (Cl. 250-211) The present invention relates to the production and control of electron emission from metals and semi-conductors into nonmetals. In particular, it relates to methods and apparatus wherein an output current may be generated by the controlled release of electrons from an electrode through lowering the potential barrier at the electrode. This method of effecting electron emission is termed high field emission which, as an electron release mechanism, is in direct contrast to thermionic emission. High field emission is operative by reducing the potential barrier while leaving unaffected the electron energy level, whereas thermionic emission serves to raise the energy level of the electrons and leaves unaffected the potential barrier.

In the present invention, the lowering of the potential barrier is accomplished by establishing a large potential difference within a relatively small distance adjacent the emitter, thereby creating a steep voltage gradient. The steep voltage gradient to effect field emission is accomplished through the controlled polarization of a dielectric material disposed in contact with the emitting electrode. By means hereinafter described, the charged particles which otherwise would exist as a uniform distribution throughout the dielectric medium may be concentrated within charged regions, hereinafter termed space charge regions, to provide the requisite field distribution.

In essence, therefore, this invention contemplates the controlled polarization of a dielectric medium to develop high field strengths or potential gradients adjacent an electrode so as to derive a current output from the electrode by high field emission.

Heretofore the concept of high field emission has had limited utilization. This is due to the fact that for external fields, an extremely high potential is required to extract electrons in this manner. The gradient theoretically predicted from the theory of metals is on the order volts per centimeter. Furthermore, such emission is not subject to control, due to breakdown effects within the medium.

It is accordingly an object of the invention to provide apparatus and devices operative by field emission phenomena to achieve new and useful results. It is likewise an object to provide such apparatus which may be operated effectively at relatively low potentials.

More specifically, it is an object of the invention to provide devices and apparatus operative by field emission, which devices may be capable of performing many of the functions of devices now operating by thermionic emission and with relatively high applied potentials.

It is likewise an object of the invention to provide novel electron emission apparatus wherein field emission of electrons is effected and controlled through the use of a dielectric medium which may be a liquid or a solid medium.

Still another object of the invention is to provide novel and useful apparatus and devices which are operative by high field emission, wherein sensitive and effective control is readily achieved without risk of destruction of the medium through inadvertent breakdown.

Another object of the invention is to provide apparatus and devices operative by field emission phenomena wherein the operation may be caused to take place over 3,017,516 Patented Jan. 16, 1962 substantial periods of time without the maintenance of continuous biasing potential.

In furtherance of the foregoing, and such other objects and features as may hereafter appear, the present invention involves the provision of novel apparatus having a wide variety of applications for sensing and control purposes, characterized by the use of polarizable dielec tric material within which an internal field may be established to effect the control of electron emission from an emitter in contact therewith.

More specifically, the invention contemplates as a feature thereof the provision of apparatus and devices operative by field emission, characterized by the use of polarizable dielectric material such as suitably prepared alkali halide crystals, wherein a steep voltage gradient may be created adjacent an electrode to effect the controlled release of electrons from the electrode into the dielectric medium.

Asa further feature of the invention, the control of the electron emission or release may be effected through a control potential, analogous to the three-element thermionic vacuum tube; by incident illumination, analogous to photo-cells; by thermal variations, or by combinations of said control functions.

While the invention is to be understood as having application broadly to a wide variety of embodiments employing controllable, non-destructive high field emission phenomena through the use of a polarizable dielectric medium, the invention is best described in relation to the use of alkali halide crystals, such as potassium bromide, having frozen-in electrons. Such crystals are colored by what are termed F centers, such coloring being effected by known techniques through exposure to alkali vapor or through the release of electrons from the cathode electrode at high temperature.

As will more fully appear hereinafter, these frozen-in electrons can be mobilized by light absorption so as to move towards the anode, leaving a voltage gradient at the cathode that can be steepened until electrons are reelasedfrom the metal electrode into the crystal. It may also be noted at this point that space charges of any desired distribution may be created by selected, localized illumination.

Other features of the invention, and the mode of operation thereof, will be set forth in conjunction with the accompanying drawings, in which FIG. 1 is a schematic representation of the ionic matrix of an alkali halide crystal colored by F centers.

FIG. 2 is a graphical representation of the field distribution in the crystal when subjected to an applied potential and before exposure to light.

FIG. 3 represents the space charge and field distribution in the crystal after contraction of the field to form a steep voltage gradient adjacent the cathode electrode.

FIG. 4 is a schematic diagram of an embodiment of the invention.

FIG. 5 are characteristic curves of photocurrent output as a function of time at various voltages under conditions of constant illumination.

FIG. 6 is a representative plot of photocurrent density as a function of light intensity.

FIG. 7 are plots showing the generation of photocurrent pulses upon exposure of the .crystal to light, as functions of dark time interval and voltages.

FIGS. 8, 9, and 10 are schematic diagrams of specific embodiments utilizing the invention.

As has been indicated, the invention will be described in terms of colored alkali halide crystals, more particularly F center crystals of potassium bromide. Such crystals may be permanently colored by exposure to alkali vapor or by other known techniques, the depth of the coloration being controllable in the treatment. An F center crystal may be defined as one in which electrons are located in the ionic matrix of the crystal replacing a minor fraction of the negative ions. These locations, when anions are missing, act as electron traps since their positive polarity attracts and holds electrons. For convenience, these electrons will be hereinafter referred to as excess or frozen-in electrons or merely electrons, and do not include those associated with a parent ion. The number of anion vacancies can be controlled within wide limits by known preparations of the crystal and constitutes a design criterion, as will hereinafter appear.

What essentially exists in the crystal then is a multiplicity of electrons in a matrix of negative and positive ions, FIG. 1, the crystal as a whole being electrically neutral as the net electron charges exactly compensate for the missing negative ions. Since the trapped electrons absorb light in a given Wavelength region and can be liberated by it through an internal photo elfeet, the fundamentals of the invention will be described in relation to this property.

Initially the alkali halide crystal, if provided with suitable electrodes to which a voltage V is applied, shows a uniform electrical field distribution, FIG. 2, across a crystal of length l. The crystal, designated as a unit by 2, contains an electron cloud of uniform density, immobilized or frozen into a compensating positive matrix across the entire body of the crystal. The designation electron cloud is used advisedly since the theoretical treatment is simplified by considering not the motion of individual electrons, but rather that of the entire electron group which may be likened to a movement of a cloud of electrons. In the original state of the crystal therefore, the electron cloud exists uniformly over the entire crystal. The use of crystal 2 in this invention will be discussed in light of these facts, with particular reference to FIG. 4 which shows the crystal arranged with source 3 of D.C. potential connected to electrodes 4 and 5 affixed to the crystal.

As a result of the field created by potential V applied across the crystal 2 in the x direction, indicated by the arrow, between cathode 4 and anode 5, a constant voltage gradient is established throughout the crystal, as shown in FIG. 2. If, then, at t (time)=0, the crystal is uniformly illuminated by light source 7, electrons become mobilized, equal in number to the quanta absorbed times the quantum yield, and drift toward the anode 5. This motion of the electron cloud toward the anode builds up a steep, adjustable cathode fall of final length doc, the electron movement leaving behind a bleached or electron-free transparent region of positive space charges in front of the cathode 4.

The total voltage V applied initially acros the geometrical length I of the crystal, would, without electron emission, appear in front of the cathode across a cathode fall of the length d )%[m] no FIG. 3 also depicts the space charge distribution and of 4.- particular interest, is the positive space charge density produced in front of the cathode.

Representative calculations can be made in accordance with Equations 1 and 3. An F center concentration of N =1 10 (electrons m and a static permittivity of 6:66 are reasonable values to assume for the alkali halides where e =dl6l6CtIlC constant of By applying 1000 volts across a crystal of 1 cm. length, the original field of uniform strength E =1 X 10 (volt in) can in theory be contracted by illumination into a cathode fall of the length (ln X10 [m] and the field strength will be raised in front of the cathode to E =2.5 lO [volt m- This final field would exceed the intrinsic breakdown strength of most alkali halide crystals. Before it is reached, however, field emission takes place from the metal electrode and arrests the further rise of the field. Thus, when the field strength at the cathode builds up beyond some selective critical value, electrons from the cathode enter the crystal by field emission. This effect keeps the gradient at the cathode lower than Equation 3 prescribes and a current will flow.

By way of example, tests conducted on alkali halide crystals such as KBr and NaCl, have revealed photocurrent outputs corresponding to the curves shown in FIG. 5. These curves are seen to deviate from a straight line relation, with the characteristic bending over rapidly to some constant current value which becomes the final current. In this connection it is useful to consult FIG. 6 showing the photocurrent density as a function of the light intensity. It is then evident that a final state is approached in which the emission from the cathode is balanced by a transconductance current through the crystal.

The manifestation of current flow in response to illumination and as a function thereof makes possible a new and useful type of photocell. Such cells, being operative by field emission, provide an extremely rapid response characteristic. Furthermore, such cells are found to possess useful sensitivity to radiation over an extremely wide range of wavelengths, from the far red to the ultraviolet.

In the embodiment heretofore described, the D.C. biasing potential is normally maintained during operation. However, for certain applications it is possible to utilize the principles of the invention in apparatus that does not depend on a maintained biasing potential applied to the electrodes. For such purposes, the crystal medium, when suitably polarized, maintains the desired field distribution to create the steep gradient necessary for field emission. That is to say, the previously described cathode fall may be maintained for substantial periods of time within the dielectric medium. As a result, the apparatus may be utilized as a sensitive detector of radiation, without requiring a maintained polarizing potential.

The persistence of the cathode fall and of field emission even after removal of the external field, can be demonstrated by direct experimental proof. When field emission exists electrons are drawn into the crystal even in the dark. They accumulate in front of the cathode, and, when the light is suddenly switched on, are released in a current pulse. FIG. 7 gives the results of tests where the current pulses, measured as a function of dark time interval and voltage, are plotted.

From the foregoing it is evident that to give rise to field emission requires that the field be concentrated in front of the cathode internally by polarization, the remainder of the dielectric retaining only so much field as needed for the transport of the electrons through the dielectric.

The preceding discussion describes the steady state condition in the buildup of the field gradient in front of the cathode. The space charge in the crystal and field gradient described adjust themselves to shorting and reapplication of a potential by a backward or forward motion of the electron cloud. Consequently, repeated use of the crystal does not affect continuity of performance due to the recurring electrical adjustment accompanying the chargings and dischargings of the crystal.

Control of the buildup is exercised by three principal factors, the dielectrics characteristics, the applied potential, and intensity of illumination.

For direct control operation, the field distribution dependence on light intensity may be said to correspond closely to the grid operation of a triode. The light intensity controls the mobility b of the electrons in the crystal and thus the field distribution. The mobility b is equal to drift velocity in the field direction per unit field strength. In the example presented the mobility is equal to the average distance E an electron travels free divided by the time t it stays trapped. The trapping time is inversely proportional to the light of intensity I, i.e.,

The factor 5 is a parameter characterizing the crystal and the frequency of the light. Since the optical absorption of the crystal is selective, the optical wavelength is also a criterion determining current output. It is possible to build a variety of absorption bands into the crystal and thus to produce elements responsive to various spectral regions.

ince the color center concentration and spectral distribution is predetermined by manufacture, the characteristics of the dielectric can be described like an electron tube by its characteristics. The output is variably dependent on the magnitude of the applied potential and intensity and spectral distribution of the electron energizing source, as described in the preceding paragraph. In this connection, however, it is to be noted that since this invention utilizes energy absorption for electron control, other means, such as controlled heat or auxiliary electrodes, may be used in place of an optical arrangement for this purpose.

In addition, inasmuch as this invention contemplates as one of its principal teachings the polarization of dielectrics to produce high field gradients, ferroelectric polarization, for example, may be used in this connection as Well :as space charge polarization. In this event, intense fields may be produced at the electrodes by the free ends of dipole chains. Control of the field distribution may then be achieved by influencing the direction and magnitude of the ferroelectric effect in the rest of the crystal by auxiliary electrodes, temperature changes, or illumination, means already disclosed in connection with space charge polarization.

In the case of control by illumination, the illumination need not be uniform throughout the crystal. It may be limited to selective regions to produce special types of field distribution as a function of the illumination pattern. Thus, superpositioning of light sources, or modulation of light, permits effective control of special and varying field distributions. The use of auxiliary electrodes of suitable configuration, or a multiplicity of electrodes selectively energizable, likewise permits local, selective control to produce special types of field distribution.

The present invention likewise contemplates the creation and utilization of field distribution in crystals by the mechanism of hole conduction, instead of by conduction by excess electrons. In such event, the distribution will be represented by an anode fall, as distinguished from the previously described cathode fall which may be caused G to occur in the vicinity of the cathode for conduction of excess electrons.

From the foregoing description, it is apparent that the invention may be used in a wide variety of embodiments and applications. FIGURES 8, 9 and 10 are representative electrical arrangements utilizing the field-emission crystals of the invention.

In FIG. 8, the dielectric medium 14, which may be an F center alkali halide crystal or other suitable polarizable dielectric material, is used as the light sensitive element to activate a relay circuit. The crystal is biased by direct current source 15, which in this embodiment is on the order of 45 volts, so that a linear field distribution appears across the crystal, as shown in FIG. 2. When light from source 17 illuminates the crystal, tribution contracts, FIG. 3, creating a voltage gradient. By suitable control of illumination, the potential gradient may be adjusted to eifect field emission and derive the required output signal. This field emission current is fed into the electrometer tube 112, changing its bias to trip the gaseous discharge tube 13 and activate the relay 8. An electrometer tube 12 is conventionally employed in the circuit to permit impedance matching between the photo-electric transducer, which is a high impedance source, and the low impedance gaseous discharge tube 13.

As has previously been mentioned, the photo-electric response of the field emission photocell is extremely rapid. It may be characterized as a primary photoeifect, since the photo-electric response is provided by the electrons directly liberated by the absorbed quanta, for example, by the F center electrons mobilized by light absorption. This may be distinguished from the secondary photo effect provided by conventional photo-electric devices, which have a relatively sluggish response. The field emission photocell is also responsive to a Wider range of the light spectrum than the usual photo-electric arrangement. It is sensitive to the infrared region as well as to the visible and ultraviolet portion of the spectrum, thereby extending the useful range of optical operation.

FIG. 9 is a photo amplifier circuit also using the principles of the invention. In this particular embodiment, the principal feature is the fact that the crystal 22 is not provided with an external bias. This is due to the fact that the crystal has been pre-polan'zed. The crystal may be pre-polarized, for example, by heating it at approximately 200 F. while maintaining a direct current poten tial across it until a steady state current flows to indicate that the crystal has been suitably polarized. The voltage and thermal sources are then removed, leaving the crystal with its internal field distribution and obviating the need for a continuous external bias.

A further feature of the circuit of FIG. 9 is the utilization of auxiliary electrodes 18. These afford means, in addition to the illumination, for controlling the field distribution, and hence the derived current, in a manner already described in connection with the circuits of FIG. 8. In this type of electrical arrangement, an output current is derived when the crystal 22 is illuminated by llght source 27, with electrodes 18 providing convenient additional control by suitable variations in the direct current supply to the electrodes. The generated field emission signal is fed through the cathode follower tube 25 to any suitable high gain electronic amplifier. The cathode follower provides impedance matching between the high 1mpedance source and the low impedance amplifier.

As an illustration of the further useful nature of the field emission crystal, FIG. 10 shows the device utilized as a rectifier. Through use of the pre-polarized crystal in the alternating current circuit, rectification occurs, and a direct current signal appears across load 30.

There has thus been described a novel and useful method and apparatus, operative by field emission, wherein electron emission may be effected by the use of a dielectric medium without requiring either a thermionic emitter or an evacuated envelope. Through the establishment of the field dis-' a steep, controllable potential gradient within the dielectric medium, potential gradients may readily be established which are effective to bring about emission of electrons from the contact electrode into the medium.

It is likewise apparent that new and useful control properties have been discovered, wherein the control of electron emission may be effected by potential variation (low potentials are adequate); by control of or response 'to illumination; and by variations in potential applied to supplementary electrodes. Furthermore, these control functions may be exercised in terms of spatial distribution within or upon the dielectric medium, so as to obtain novel and useful control functions.

Accordingly, it will be understood that the invention has been described in terms of illustrative embodiments only, and that the invention contemplates a wide variety of embodiments and applications of field emission phenomena wherein electron emission is obtained by steep potential gradients created within a dielectric medium through electron mobilization and migration to produce a concentrated, controllable field adjacent an electrode.

I claim as my invention:

1. Apparatus for the production and control of electron emission comprising a polarizable dielectric medium, electrodes in conductive contact therewith, said dielectric medium having an internal field distribution to provide a steep potential gradient in the vicinity of the first of said electrodes, said potential gradient being sufficiently steep to effect the emission of electrons from said first electrode into said dielectric medium by field emission independent of a sustained applied voltage across said electrodes.

2. The combination defined in claim 1 which includes a source of direct voltage and means for applying said direct voltage to said electrodes.

3. The combination defined in claim 1 which includes supplementary electrodes for modifying the field distribution within said dielectric medium.

4. The combination defined in claim 1 which includes means for illuminating said dielectric medium to modify the field distribution therein.

5. Apparatus for the production and control of elec tron emission comprising a polarizable dielectric medium, electrodes in conductive contact therewith, said dielectric medium having an internal field distribution to provide a steep potential gradient in the vicinity of the first of said electrodes, said potential gradient being sufficiently steep to effect the emission of electrons from said first electrode into said dielectric medium, a source of direct voltage, means for applying said direct voltage to said electrodes, and means for illuminating said dielectric medium to modify the field distribution therein.

6. Apparatus for the production and control of electron emission comprising a polarizable dielectric medium, electrodes in conductive contact therewith, said dielectric medium having an internal field distribution to provide a steep potential gradient in the vicinity of the first of said electrodes, said potential gradient being sufficiently steep to effect the emission of electrons from said first electrode into said dielectric medium, a source of direct voltage, means for applying said direct voltage to said electrodes, and supplementary electrodes for modifying the field distribution within said dielectric medium.

7. Apparatus for the production and control of electron ernission comprising a polarizable dielectric medium, electrodes in conductive contact therewith, said medium having an excess of charges disposed adjacent a first of said electrodes to form a steep potential gradient within the medium, said potential gradient being sufficiently steep to effect the emission of electrons into said dielectric medium by field emission from said first electrode independent of a sustained voltage applied across said electrodes.

8. The combination defined in claim 7 which includes a source of direct voltage and means for applying said direct voltage to said electrodes.

9. Apparatus for the production and control of electron emission comprising a polarizable dielectric medium, said medium being an alkali halide crystal, electrodes in conductive contact therewith, said crystal having an excess of charges disposed adjacent a first electrode attached thereto to form a steep potential gradient within said crystal, said potential gradient being sutficiently steep to effect the emission of electrons from said first electrode into said crystal by field emission, independent of a sustained voltage applied across said electrodes.

10. The combination defined in claim 9 in which said crystal is an additively colored alkali halide crystal.

11. The combination defined in claim 9 in which said alkali halide crystal is an F center colored crystal of potassium bromide.

12. The combination defined in claim 9 which includes a source of direct voltage, and means for applying said direct voltage to said electrodes.

13. A photocell operative by field emission comprising a polarizable dielectric medium, electrodes in conductive contact therewith, said dielectric having an excess of charges adjacent a first of said electrodes to establish a steep potential gradient within the medium, said potential gradient being sufiiciently steep to effect emission of electrons from said first electrode into said dielectric medium by field emission, independent of a sustained applied voltage across said electrodes, and current responsive apparatus connected to the electrodes to respond to currents flowing between the electrodes upon exposure of said dielectric medium to illumination.

14. A method for making a device capable of producing and controlling electron emission which comprises steps of applying a potential difference to a polarizable dielectric medium, exposing said medium to illumination to mobilize electrons contained therein, and thereby effect a concentration of charges adjacent one electrode, and controlling the potential applied to the electrodes and the illumination of said dielectric medium to provide a potential gradient adjacent said one electrode of sufiicient steepness to effect the emission of electrons from said electrode into said dielectric medium by field emission independent of a sustained applied voltage across said electrodes.

References Cited in the file of this patent UNITED STATES PATENTS 1,827,016 Joife Oct. 13, 1931 2,330,171 Rosenthal Sept. 21, 1943 2,713,116 Raibourn July 12, 1955 2,759,124 Willis Aug. 14, 1956 2,780,731 Miller Feb. 5, 1957 2,792,752 Moncrielf-Yeates et al. May 21, 1957 OTHER REFERENCES Rosenthal: Proceedings of the I.R.E., vol. 28; 1940; pp. 203412.

Mott & Gurney: Electronic Processes in Ionic Crystals; 2nd edition 1948; Oxford University Press.

Von Hippel et al.: Laboratory for Insulation Research; Technical Report 1959; Massachusetts Institute of Technology; O.N.R. Contracts N50ri-07801 and NSOri- 07858; February 1953.

Von Hippel et al.: Physical Review; volume 91; No. 3; August 1, 1953, PP. 568-579. 

